Nine out of 10 Americans consume some type of caffeine regularly, making it the most popular behavior-altering drug. For most people, moderate doses of caffeine (about two to three cups of brewed coffee a day) are not harmful. However, heavy daily caffeine use (about four to seven cups of coffee) can cause restlessness, anxiety, irritability, muscle tremors, sleeplessness, headaches, nausea, diarrhea or other gastrointestinal problems and abnormal heart rhythms.